Generic seat belt retractors are used for example in motor vehicles for the safety belts of the rear seats. There is the general problem about rear seats that the distance available for the forward displacement of children or smaller adults, as for example the 5th percentile-sized woman in a frontal collision, shall be utilized to the maximum, whereas for the heavy passenger the same safety belt must prevent a maximum belt webbing extraction length from being exceeded in a collision. Therefore, the load limiting device of such a seat belt retractor generally comprises two load limiting elements, by the time-shifted activation of which the load limiting level can be switched from a lower load limiting level to a higher load limiting level. Such a load limiting characteristic is also referred to as progressive load limiting.
From DE 103 27 204 B4 for example a seat belt retractor is known, in which a torsion rod is located between the profile head and the belt shaft, which torsion rod, during the load limited belt webbing extraction, is twisted around its own axis in an energy absorbing manner at the same time being subjected to plastic deformation. Furthermore, a metal band acting parallel to the torsion rod is provided, which metal band with one end is firmly hinged at a threaded bushing and in its course is pulled through a baffle fixed on the profile head. The threaded bushing being fixed at the beginning of the load limiting operation engages with an outer thread into an inner thread on the belt shaft. During the load limiting operation, at first, the torsion rod is twisted at the same time being subjected to plastic deformation, wherein the belt shaft rotates in relation to the threaded bushing. In doing so, the threaded bushing is pulled to the dead stop position so that the metal band firmly connected to the threaded bushing with one end is subsequently pulled through the baffle. The deformation energy or work resulting from the metal band being pulled through the baffle adds to the load limiting level effectuated by the torsion of the torsion rod so that the load limiting level is thereby switched from a lower to a higher load limiting level. However, it is a disadvantage of the proposed seat belt retractor that the progressive load limiting characteristic is achieved by the load limiting elements being activated simultaneously, and that the load limiting elements do not work independent of each other. Furthermore, the beginning of the high load limiting level depends on the time when the threaded bushing is pulled to the dead stop position. The time when the threaded bushing is pulled to the dead stop position depends on the distance between the threaded bushing and the belt shaft and on the pitch of the thread. As the available installation space is highly limited, it is desirable to use a thread having a pitch as small as possible resulting in the disadvantage that even minor deviations of the distance lead to substantial deviations of the rotation angle, at which the load limiting level is switched, from the predetermined rotation angle.
It is the object of the invention to provide a seat belt retractor comprising a load limiting device with a progressive load limiting characteristic, in which seat belt retractor it shall be possible to dimension the load limiting levels independent of each other and to determine the time of switching in relation to the load limited belt webbing extraction movement with a precision as high as possible.